The present invention describes radiation-curable coating systems based on aqueous polyurethane dispersions, a process for the preparation thereof, the use of the coating systems as lacquers and/or adhesives, and objects and substrates provided with these lacquers and/or adhesives.
Radiation-curable aqueous coating systems based on polyurethane polymers are used in the coating, inter alia, of wood, plastics and leather and are distinguished by a large number of positive properties, such as good resistance to chemicals and mechanical stability. A particular advantage is the split-second curing of the polyurethane top layer by crosslinking of the ethylenic double bonds contained in the polymer with the aid of high-energy radiation.
For many uses, such as e.g. in the lacquering of wood/furniture or plastics, marked physical surface drying after the water has been allowed to evaporate and before the radiation curing plays an exceptionally important role. Thus, coatings which, after the water has been allowed to evaporate, are touch dry and non-blocking and are not yet radiation cured can already be sanded, stacked and subjected to mechanical stress in diverse ways.
For pigmented lacquers a further significant advantage results if the coatings are already touch dry and non-blocking also in the state before radiation curing. Pigments, such as e.g. titanium dioxide, scatter and absorb UV radiation, and at a high pigment content can ensure that the radiation-induced polymerization proceeds incompletely in lower layers of the lacquer. In systems which are still tacky before radiation curing, this means that after radiation curing there is a soft or still viscous layer under the cured upper layer. Poor adhesion to the substrate and poor resistances to chemicals and colouring agents are the result. If the lower layer of a pigmented lacquer is firm per se and not tacky in spite of inadequate radiation curing, both the adhesion and the resistances become better.
The requirements of modern coating systems are very diverse. Thus not only is it of advantage if radiation-curable coating systems are non-blocking and touch dry after physical drying, rather after radiation curing a film should be obtained which is distinguished by high chemical resistances and good resistance to mechanical stress.
DE-A 2912574 describes water-dispersible urethane oligomers which are terminated by acrylic groups and are built up from polyisocyanates, di- or trimethylolcarboxylic acids, polytetramethylene diols, polycaprolactone polyols and monohydroxy-functional acrylates. These urethane oligomers are used as a textile coating, where a particular flexibility, elasticity and tensile strength are important.
EP-A 98752 describes a process for the preparation of aqueous polyurethane dispersions, which are obtained by reaction of diols with ionizable groups, polyester polyols, polyether polyols, diisocyanates and monohydroxy-functional acrylates.
EP-A 181486 claims aqueous dispersions of oligourethanes, which are crosslinkable by UV radiation, for the production of high-gloss lacquer coatings on leather, comprising polyether, polyester, polylactone or polycarbonate diols, anionic or nonionic compounds which are mono- or difunctional with respect to isocyanate groups, diisocyanates, low molecular weight diamines and/or diols and monohydroxy-functional acrylates. These systems are very flexible and elastic.
Aqueous emulsions based on ionic urethane-urea acrylates are described in EP-A 209684, which are characterized in that they comprise cycloaliphatic polyisocyanates, hydroxyalkyl acrylates, polyester polyol, polyether polyols and sodium salts of aminocarboxylic acids and are prepared by an acetone process.
EP-A 392352 describes aqueous dispersions of polyurethanes, which are crosslinkable with high-energy radiation, for coating flexible substrates. These are built up from polyisocyanates, high molecular weight polyols, low molecular weight polyols, polyamines, amino alcohols, an OH- or NH-functional compound having an ionizable group, monofunctional polyether polyols and compounds having ethylenically unsaturated groups and at least one hydroxyl group.
The five patents listed above are distinguished in that they provide systems for flexible and elastic coatings, with textiles as the preferred substrate. The polyester polyols built up from long-chain aliphatic polyols and/or di-acids are used as the flexibilizing builder component. The physical surface drying of the systems described is inadequate, and the resistances to chemicals are likewise not adequate. This manifests itself in particular in pigmented formulations.
EP-A 704469 describes water-dispersible, radiation-curable polyurethanes comprising a) polyisocyanates, b) polyester polyols, c) low molecular weight polyols, d) compounds having at least one group which is reactive towards isocyanate and at least one carboxyl group, e) a compound having at least one group which is reactive towards isocyanate and at least one copolymerizable unsaturated group and optionally f) an at least difunctional amine. In this context, at least components a), b) and e) must be reacted in one stage for the preparation of the polyurethane. Since these systems are employed in particular in textile coating, they must be very flexible.
Although it is prior art to build up radiation-curable polyurethane dispersions with the aid of polyester polyols, polyester polyols have thus hitherto preferably been used as flexibilizing builder components in that the polyesters were built up from long-chain and aliphatic polyols and/or di-acids. Such flexibilizing polyester polyols as units in a radiation-curable polyurethane dispersion lead to a low degree of physical drying and to inadequate resistances to colouring agents and solvents, especially in pigmented formulations.
The object was to provide radiation-curable coating systems which make rapid physical drying possible, are highly non-blocking after drying and make films which are very hard and resistant to chemicals possible after radiation curing. However, the films should not be brittle and should still be sufficiently flexible. This should apply to clear and pigmented lacquers.
It has been found, surprisingly, that radiation-curable aqueous dispersions of polyurethane acrylates give non-blocking coatings after a short drying time if these contain polyester polyols based on aromatic di- and/or tricarboxylic acids and aliphatic diols having 2 to 4 carbon atoms or aliphatic triols. The films of these dispersions, both in clear lacquer and as pigmented lacquer, moreover achieve a high pendulum hardness after radiation curing, and prove to be very resistant to chemicals and colouring agents.